SAC Single-attached concentrator. FDDI or CDDI concentrator that connects to the network by being cascaded from the master port of another FDDI or CDDI concentrator. safety ground wire Circuit wire that connects to a local earth ground and the chassis of an electrical appliance or device via an electrical outlet and plug. It is used to ensure that no voltage potential exists between the chassis of the electrical device and the earth ground. sag Any decrease of below 80% in the normal voltage carried by a power line. A sag is sometimes referred to as a brownout. See surge, spike, and oscillation. sampling rate Rate at which samples of a particular waveform amplitude are taken. SAP 1.) Service access point. Field defined by the IEEE 802.2 specification that is part of an address specification. Thus, the destination plus the DSAP define the recipient of a packet. The same applies to the SSAP. See also DSAP and SSAP. 2.) Service Advertisement Protocol. IPX protocol that provides a means of informing network clients, via routers and servers, of available network resources and services. See also IPX. SAR Segmentation and reassembly. One of the two sublayers of the AAL CPCS, responsible for dividing (at the source) and reassembling (at the destination) the PDUs passed from the CS. The SAR sublayer takes the PDUs processed by the CS and, after dividing them into 48-byte pieces of payload data, passes them to the ATM layer for further processing. See also AAL, ATM layer, CPCS, CS, and SSCS. SAS Single attachment station. Device attached only to the primary ring of an FDDI ring. Also known as a Class B station. Compare with DAS. See also FDDI. satellite communication Use of orbiting satellites to relay data between multiple earth-based stations. Satellite communications offer high bandwidth and a cost that is not related to distance between earth stations, long propagation delays, or broadcast capability. SBus Bus technology used in Sun SPARC-based workstations and servers. The SBus specification has been adopted by the IEEE as a new bus standard. SCR Sustainable cell rate. Parameter defined by the ATM Forum for ATM traffic management. For VBR connections, SCR determines the long-term average cell rate that can be transmitted. See also VBR. SCTE Serial clock transmit external. Timing signal that DTE echoes to DCE to maintain clocking. SCTE is designed to compensate for clock phase shift on long cables. When the DCE device uses SCTE instead of its internal clock to sample data from the DTE, it is better able to sample the data without error even if there is a phase shift in the cable. See also phase shift. SDH Synchronous Digital Hierarchy. European standard that defines a set of rate and format standards that are transmitted using optical signals over fiber. SDH is similar to SONET, with a basic SDH rate of 155.52 Mbps, designated at STM-1. See also SONET and STM-1. SDLC Synchronous Data Link Control. SNA data link layer communications protocol. SDLC is a bit-oriented, full-duplex serial protocol that has spawned numerous similar protocols, including HDLC and LAPB. See also HDLC and LAPB. SDLC broadcast Feature that allows a Cisco router that receives an all-stations broadcast on a virtual multidrop line to propagate the broadcast to each SDLC line that is a member of the virtual multidrop line. SDLC Transport Cisco router feature with which disparate environments can be integrated into a single, high-speed, enterprise-wide network. Native SDLC traffic can be passed through point-to-point serial links with other protocol traffic multiplexed over the same links. Cisco routers can also encapsulate SDLC frames inside IP datagrams for transport over arbitrary (non-SDLC) networks. Replaces proxy polling. See also proxy polling. SDLLC Feature that performs translation between SDLC and IEEE 802.2 type 2. SDSU SMDS DSU. DSU for access to SMDS via HSSIs and other serial interfaces. SDU Service data unit. Unit of information from an upper-layer protocol that defines a service request to a lower-layer protocol. SEAL Simple and efficient AAL. Scheme used by AAL5 in which the SAR sublayer segments CS PDUs without adding additional fields. See also AAL, AAL5, CS, and SAR. secondary See secondary station. secondary ring One of the two rings making up an FDDI or CDDI ring. The secondary ring is usually reserved for use in the event of a failure of the primary ring. Compare to primary ring. secondary station In bit-synchronous data link layer protocols such as HDLC, a station that responds to commands from a primary station. Sometimes referred to simply as a secondary. See also primary station. security management One of five categories of network management defined by ISO for management of OSI networks. Security management subsystems are responsible for controlling access to network resources. See also accounting management, configuration management, fault management, and performance management. seed router Responds to configuration queries from nonseed routers on its connected AppleTalk network, allowing those routers to confirm or modify their configurations accordingly. See also nonseed router. segment 1.) Section of a network that is bounded by bridges, routers, or switches. 2.) In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters. 3.) Term used in the TCP specification to describe a single transport layer unit of information. The terms datagram, frame, message, and packet are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles. segmentation and reassembly See SAR. sequence number Number used to ensure correct sequencing of the arriving data. Sequenced Packet Exchange See SPX. Sequenced Packet Protocol See SPP. Sequenced Routing Update Protocol See SRTP. serial clock transmit external See SCTE. Serial Interface Processor See SIP. Serial Line Internet Protocol See SLIP. serial transmission Method of data transmission in which the bits of a data character are transmitted sequentially over a single channel. Compare with parallel transmission. serial tunnel See STUN. server Node or software program that provides services to clients. See also back end, client, and front end. Server Message Block See SMB. service access point See SAP. Service Advertisement Protocol See SAP. service data unit See SDU. service password-encryption Command that allows further protection. service point Interface between non-SNA devices and NetView that sends alerts from equipment unknown to the SNA environment. Service Profile Identifier See SPID. service specific convergence sublayer See SSCS. session 1.) Related set of communications transactions between two or more network devices. 2.) In SNA, a logical connection enabling two NAUs to communicate. session layer Layer 5 of the OSI reference model. This layer establishes, manages, and terminates sessions between applications and manages data exchange between presentation layer entities. Corresponds to the data flow control layer of the SNA model. See also application layer, data link layer, network layer, physical layer, presentation layer, and transport layer. SF Super Frame. Common framing type used on T1 circuits. SF consists of 12 frames of 192 bits each, with the 193rd bit providing error checking and other functions. SF has been superseded by ESF, but is still widely used. Also called D4 framing. See also ESF. SGMP Simple Gateway Monitoring Protocol. Network management protocol that was considered for Internet standardization and later evolved into SNMP. Documented in RFC 1028. See also SNMP. shaping See traffic shaping. shielded cable Cable that has a layer of shielded insulation to reduce EMI. shielded twisted-pair See STP. shortest path first algorithm See SPF. shortest-path routing Routing that minimizes distance or path cost through application of an algorithm. show access-lists Command that displays the contents of all access lists. show flash 1.) Command used to verify that you have sufficient memory on your system for the Cisco IOS software you want to load. 2.) Command used to learn the name of the system image file. show running-config Command used to display the current configuration in RAM. show ip interface Command that displays the status and global parameters associated with an interface. show ip protocol 1.) Command that displays values about routing timers and network information associated with the entire router. 2.) Command that displays parameters, filters, and network information about the entire router. show ip route Command that displays the contents of an IP routing table. show startup-config Command used to display the saved configuration. signaling Process of sending a transmission signal over a physical medium for purposes of communication. signaling packet Generated by an ATM-connected device that wants to establish a connection with another such device. The signaling packet contains the ATM NSAP address of the desired ATM endpoint, as well as any QOS parameters required for the connection. If the endpoint can support the desired QOS, it responds with an accept message, and the connection is opened. See also QOS. Signaling System number 7 See SS7. signal injector Device used to measure attenuation of a signal on a network. signal quality error See SQE. signal reference ground Reference point used by computing devices to measure and compare incoming digital signals to.Reference point used by computing devices to measure and compare incoming digital signals to. silicon switching Switching based on the SSE, which allows the processing of packets independent of the SSP (Silicon Switch Processor) system processor. Silicon switching provides high-speed, dedicated packet switching. See also SSE and SSP (Silicon Switch Processor). silicon switching engine See SSE. Silicon Switch Processor See SSP. simple and efficient AAL See SEAL. Simple Gateway Monitoring Protocol See SGMP. Simple Mail Transfer Protocol See SMTP. Simple Multicast Routing Protocol See SMRP. Simple Network Management Protocol See SNMP. simplex Capability for data transmission in only one direction between a sending station and a receiving station. Compare with full duplex and half duplex. single-attached concentrator See SAC. single attachment station See SAS. single-mode fiber Fiber-optic cabling with a narrow core that allows light to enter only at a single angle. Such cabling has higher bandwidth than multimode fiber, but requires a light source with a narrow spectral width (for example, a laser). Also called monomode fiber. See also multimode fiber. single-route explorer packet See spanning explorer packet. single-vendor network Network using equipment from only one vendor. Single-vendor networks rarely suffer compatibility problems. See also multivendor network. SIP 1.) SMDS Interface Protocol. Used in communications between CPE and SMDS network equipment. Allows the CPE to use SMDS service for high-speed WAN internetworking. Based on the IEEE 802.6 DQDB standard. See also DQDB. 2.) Serial Interface Processor. Obsolete interface processor for Cisco 7000 series routers that provided either two or four channel-independent ports for synchronous serial connections at speeds from 2.4 Kbps to 4 Mbps. The SIP has been replaced by the FSIP. Sometimes called SX-SIP or Pre-FSIP. See also FSIP. sliding window Refers to the fact that the window size is negotiated dynamically during the TCP session. sliding window flow control Method of flow control in which a receiver gives transmitter permission to transmit data until a window is full. When the window is full, the transmitter must stop transmitting until the receiver advertises a larger window. TCP, other transport protocols, and several data link layer protocols use this method of flow control. SLIP Serial Line Internet Protocol. Standard protocol for point-to-point serial connections using a variation of TCP/IP. Predecessor of PPP. See also CSLIP and PPP. slotted ring LAN architecture based on a ring topology in which the ring is divided into slots that circulate continuously. Slots can be either empty or full, and transmissions must start at the beginning of a slot. slow switching Packet processing performed at process level speeds, without the use of a route cache. Contrast with fast switching. SMAC Source MAC. MAC address specified in the Source Address field of a packet. Compare with DMAC. See also MAC address. SMB Server Message Block. File-system protocol used in LAN Manager and similar NOSs to package data and exchange information with other systems. SMDS Switched Multimegabit Data Service. High-speed, packet-switched, datagram-based WAN networking technology offered by the telephone companies. See also CBDS. SMDS Interface Protocol See SIP. SMI Structure of Management Information. Document (RFC 1155) specifying rules used to define managed objects in the MIB. See also MIB. smoothing See traffic shaping. SMRP Simple Multicast Routing Protocol. Specialized multicast network protocol for routing multimedia data streams on enterprise networks. SMRP works in conjunction with multicast extensions to the AppleTalk protocol. SMT Station Management. ANSI FDDI specification that defines how ring stations are managed. SMTP Simple Mail Transfer Protocol. Internet protocol providing electronic mail services. SNA Systems Network Architecture. Large, complex, feature-rich network architecture developed in the 1970s by IBM. Similar in some respects to the OSI reference model, but with a number of differences. SNA is essentially composed of seven layers. See data flow control layer, data link control layer, path control layer, physical control layer, presentation services layer, transaction services layer, and transmission control layer. SNA Distribution Services See SNADS. SNA Network Interconnection See SNI. SNADS SNA Distribution Services. Consists of a set of SNA transaction programs that interconnect and cooperate to provide asynchronous distribution of information between end users. One of three SNA transaction services. See also DDM and DIA. SNAP Subnetwork Access Protocol. Internet protocol that operates between a network entity in the subnetwork and a network entity in the end system. SNAP specifies a standard method of encapsulating IP datagrams and ARP messages on IEEE networks. The SNAP entity in the end system makes use of the services of the subnetwork and performs three key functions: data transfer, connection management, and QOS selection. SNI 1.) Subscriber Network Interface. Interface for SMDS-based networks that connects CPE and an SMDS switch. See also UNI. 2.) SNA Network Interconnection. IBM gateway connecting multiple SNA networks. SNMP Simple Network Management Protocol. Network management protocol used almost exclusively in TCP/IP networks. SNMP provides a means to monitor and control network devices, and to manage configurations, statistics collection, performance, and security. See also SGMP and SNMP2. SNMP communities Authentication scheme that enables an intelligent network device to validate SNMP requests from sources such as the NMS. A LightStream 2020 ATM switch, for example, responds only to SNMP requests that come from members of known communities and that have the access privileges required for that request. See also SNMP. SNMP2 SNMP Version 2. Version 2 of the popular network management protocol. SNMP2 supports centralized as well as distributed network management strategies, and includes improvements in the SMI, protocol operations, management architecture, and security. See also SNMP. SNPA Subnetwork point of attachment. A data link layer address (such as an Ethernet address, X.25 address, or Frame Relay DLCI address). SNPA addresses are used to configure a CLNS route for an interface. socket Software structure operating as a communications end point within a network device. soc ket number An 8-bit number that identifies a socket. A maximum of 254 different socket numbers can be assigned in an AppleTalk node. SONET Synchronous Optical Network. High-speed (up to 2.5 Gbps) synchronous network specification developed by Bellcore and designed to run on optical fiber. STS-1 is the basic building block of SONET. Approved as an international standard in 1988. See also SDH, STS-1, and STS-3c. source address Address of a network device that is sending data. See also destination address. source and destination IP addresses Field within an IP datagram that indicates the 32-bit source and destination IP addresses. source MAC See SMAC. source port Number of the calling port. source-route bridging See SRB. source-route translational bridging See SR/TLB. source-route transparent bridging See SRT. source service access point See SSAP. Southeastern Universities Research Association Network See SURAnet. SP Switch Processor. Cisco 7000-series processor module that acts as the administrator for all CxBus activities. Sometimes called ciscoBus controller. See also CxBus. SPAN Switched Port Analyzer. Feature of the Catalyst 5000 switch that extends the monitoring abilities of existing network analyzers into a switched Ethernet environment. SPAN mirrors the traffic at one switched segment onto a predefined SPAN port. A network analyzer attached to the SPAN port can monitor traffic from any of the other Catalyst switched ports. span Full-duplex digital transmission line between two digital facilities. spanning explorer packet Follows a statically configured spanning tree when looking for paths in an SRB network. Also known as a limited-route explorer packet or a single-route explorer packet. See also all-routes explorer packet, explorer packet, and local explorer packet. spanning tree Loop-free subset of a network topology. See also spanning-tree algorithm and Spanning-Tree Protocol. spanning-tree algorithm Algorithm used by the Spanning-Tree Protocol to create a spanning tree. Sometimes abbreviated STA. See also spanning tree and Spanning-Tree Protocol. Spanning-Tree Protocol Bridge protocol that utilizes the spanning-tree algorithm, enabling a learning bridge to dynamically work around loops in a network topology by creating a spanning tree. Bridges exchange BPDU messages with other bridges to detect loops, and then remove the loops by shutting down selected bridge interfaces. Refers to both the IEEE 802.1 Spanning-Tree Protocol standard and the earlier Digital Equipment Corporation Spanning-Tree Protocol upon which it is based. The IEEE version supports bridge domains and allows the bridge to construct a loop-free topology across an extended LAN. The IEEE version is generally preferred over the Digital version. Sometimes abbreviated STP. See also BPDU, learning bridge, MAC address learning, spanning tree, and spanning-tree algorithm. sparse mode PIM See PIM sparse mode. speed matching Feature that provides sufficient buffering capability in a destination device to allow a high-speed source to transmit data at its maximum rate, even if the destination device is a lower-speed device. SPF Shortest path first algorithm. Routing algorithm that iterates on length of path to determine a shortest-path spanning tree. Commonly used in link-state routing algorithms. Sometimes called Dijkstra's algorithm. See also link state routing algorithm. SPID Service Profile Identifier. Number that some service providers use to define the services to which an ISDN device subscribes. The ISDN device uses the SPID when accessing the switch that initializes the connection to a service provider. spike Any power impulse lasting between .5 and 100 microseconds and possessing an amplitude over 100 % of peak power line voltage. See surge, sag, and oscillation. split-horizon updates Routing technique in which information about routes is prevented from exiting the router interface through which that information was received. Split-horizon updates are useful in preventing routing loops. spoofing 1.) Scheme used by Cisco routers to cause a host to treat an interface as if it were up and supporting a session. The router spoofs replies to keepalive messages from the host in order to convince that host that the session still exists. Spoofing is useful in routing environments such as DDR, in which a circuit-switched link is taken down when there is no traffic to be sent across it in order to save toll charges. See also DDR. 2.) The act of a packet illegally claiming to be from an address from which it was not actually sent. Spoofing is designed to foil network security mechanisms such as filters and access lists. spooler Application that manages requests or jobs submitted to it for execution. Spoolers process the submitted requests in an orderly fashion from a queue. A print spooler is a common example of a spooler. SPP Sequenced Packet Protocol. Provides reliable, connection-based, flow-controlled packet transmission on behalf of client processes. Part of the XNS protocol suite. SPX Sequenced Packet Exchange. Reliable, connection-oriented protocol that supplements the datagram service provided by network layer (Layer 3) protocols. Novell derived this commonly used NetWare transport protocol from the SPP of the XNS protocol suite. SQE Signal quality error. Transmission sent by a transceiver back to the controller to let the controller know whether the collision circuitry is functional. Also called heartbeat. SRAM Type of RAM that retains its contents for as long as power is supplied. SRAM does not require constant refreshing, like DRAM. Compare with DRAM. SRB Source-route bridging. Method of bridging originated by IBM and popular in Token Ring networks. In a SRB network, the entire route to a destination is predetermined, in real time, prior to the sending of data to the destination. Contrast with transparent bridging. SRT Source-route transparent bridging. IBM bridging scheme that merges the two most prevalent bridging strategies, SRB and transparent bridging. SRT employs both technologies in one device to satisfy the needs of all ENs. No translation between bridging protocols is necessary. Compare with SR/TLB. SR/TLB Source-route translational bridging. Method of bridging where source-route stations can communicate with transparent bridge stations with the help of an intermediate bridge that translates between the two bridge protocols. Compare with SRT. SRTP Sequenced Routing Update Protocol. Protocol that assists VINES servers in finding neighboring clients, servers, and routers. See also RTP (Routing Table Protocol). SS7 Signaling System number 7. Standard CCS system used with BISDN and ISDN. Developed by Bellcore. See also CCS. SSAP Source service access point. The SAP of the network node designated in the Source field of a packet. Compare to DSAP. See also SAP (service access point). SSCP System services control points. Focal points within an SNA network for managing network configuration, coordinating network operator and problem determination requests, and providing directory services and other session services for network end users. SSCP-PU session Session used by SNA to allow an SSCP to manage the resources of a node through the PU. SSCPs can send requests to, and receive replies from, individual nodes in order to control the network configuration. SSCS Service specific convergence sublayer. One of the two sublayers of any AAL. SSCS, which is service dependent, offers assured data transmission. The SSCS can be null as well, in classical IP over ATM or LAN emulation implementations. See also AAL, ATM layer, CPCS, CS, and SAR. SSE Silicon switching engine. Routing and switching mechanism that compares the data link or network layer header of an incoming packet to a silicon-switching cache, determines the appropriate action (routing or bridging), and forwards the packet to the proper interface. The SSE is directly encoded in the hardware of the SSP (Silicon Switch Processor) of a Cisco 7000 series router. It can therefore perform switching independently of the system processor, making the execution of routing decisions much quicker than if they were encoded in software. See also silicon switching and SSP (Silicon Switch Processor). SSP 1.) Silicon Switch Processor. High-performance silicon switch for Cisco 7000 series routers that provides distributed processing and control for interface processors. The SSP leverages the high-speed switching and routing capabilities of the SSE to dramatically increase aggregate router performance, minimizing performance bottlenecks at the interface points between the router and a high-speed backbone. See also silicon switching and SSE. 2.) Switch-to-Switch Protocol. Protocol specified in the DLSw standard that routers use to establish DLSw connections, locate resources, forward data, and handle flow control and error recovery. See also DLSw. STA See spanning-tree algorithm. stack See protocol stack. standard Set of rules or procedures that are either widely used or officially specified. See also de facto standard. standby monitor Device placed in standby mode on a Token Ring network in case an active monitor fails. See also active monitor and ring monitor. StarLAN CSMA/CD LAN, based on IEEE 802.3, developed by AT&T. star topology LAN topology in which end points on a network are connected to a common central switch by point-to-point links. A ring topology that is organized as a star implements a unidirectional closed-loop star, instead of point-to-point links. Compare with bus topology, ring topology, and tree topology. start-stop transmission See asynchronous transmission. static electricity Unpredictable electrical charges in the atmosphere that interfere with radio reception, computer networking, and the like. static route Route that is explicitly configured and entered into the routing table. Static routes take precedence over routes chosen by dynamic routing protocols. Station Management See SMT. statistical multiplexing Technique whereby information from multiple logical channels can be transmitted across a single physical channel. Statistical multiplexing dynamically allocates bandwidth only to active input channels, making better use of available bandwidth and allowing more devices to be connected than with other multiplexing techniques. Also referred to as statistical time-division multiplexing or stat mux. Compare with ATDM, FDM, and TDM. statistical time-division multiplexing See statistical multiplexing. stat mux See statistical multiplexing. STM-1 Synchronous Transport Module level 1. One of a number of SDH formats that specifies the frame structure for the 155.52-Mbps lines used to carry ATM cells. See also SDH. store and forward packet switching Packet-switching technique in which frames are completely processed before being forwarded out the appropriate port. This processing includes calculating the CRC and checking the destination address. In addition, frames must be temporarily stored until network resources (such as an unused link) are available to forward the message. Contrast with cut-through packet switching. STP 1. Shielded twisted-pair. Two-pair wiring medium used in a variety of network implementations. STP cabling has a layer of shielded insulation to reduce EMI. Compare with UTP. See also twisted pair. 2. See Spanning-Tree Protocol. StreamView network management Cisco suite of SNMP-based network management tools used in conjunction with the LightStream 2020 ATM switch. The StreamView suite includes three GUI-driven applications: a configuration program (the configurator), a network topology map (the topology map), and a node monitoring program (the monitor); and a command-line interface: the CLI. See also CLI, configurator, monitor, and topology map. Structure of Management Information See SMI. STS-1 Synchronous Transport Signal level 1. Basic building block signal of SONET, operating at 51.84 Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84 Mbps. See also SONET. STS-3c Synchronous Transport Signal level 3, concatenated. SONET format that specifies the frame structure for the 155.52-Mbps lines used to carry ATM cells. See also SONET. stub area OSPF area that carries a default route, intra-area routes, and interarea routes, but does not carry external routes. Virtual links cannot be configured across a stub area, and they cannot contain an ASBR. Compare to non-stub area. See also ASBR and OSPF. stub network Network that has only a single connection to a router. STUN Serial tunnel. Router feature allowing two SDLC- or HDLC-compliant devices to connect to one another through an arbitrary multiprotocol topology (using Cisco routers) rather than through a direct serial link. subarea Portion of an SNA network that consists of a subarea node and any attached links and peripheral nodes. subarea node SNA communication controller or host that handles complete network addresses. subchannel In broadband terminology, a frequency-based subdivision creating a separate communications channel. subinterface One of a number of virtual interfaces on a single physical interface. subnet See subnetwork. subnet address Portion of an IP address that is specified as the subnetwork by the subnet mask. See also IP address, subnet mask, and subnetwork. subnet mask 32-bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. Sometimes referred to simply as mask. See also address mask and IP address. subnetwork 1.) In IP networks, a network sharing a particular subnet address. Subnetworks are networks arbitrarily segmented by a network administrator in order to provide a multilevel, hierarchical routing structure while shielding the subnetwork from the addressing complexity of attached networks. Sometimes called a subnet. See also IP address, subnet address, and subnet mask. 2.) In OSI networks, a collection of ESs and ISs under the control of a single administrative domain and using a single network access protocol. Subnetwork Access Protocol See SNAP. subnetwork point of attachment See SNPA. Subscriber Network Interface See SNI. subvector A data segment of a vector in an SNA message. A subvector consists of a length field, a key that describes the subvector type, and subvector specific data. Super Frame See SF. supervisory processor See RP (Route Processor). SURAnet Southeastern Universities Research Association Network. Network connecting universities and other organizations in the Southeastern United States. SURAnet, originally funded by the NSF and a part of the NSFNET, is now part of BBN Planet. See also BBN Planet, NSF, and NSFNET. surge Any voltage increase above 110 % of the normal voltage carried by a power line. See sag, spike, and oscillation. sustainable cell rate See SCR. SVC Switched virtual circuit. Virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic. Called a switched virtual connection in ATM terminology. Compare with PVC. switch 1.) Network device that filters, forwards, and floods frames based on the destination address of each frame. The switch operates at the data link layer of the OSI model. 2.) General term applied to an electronic or mechanical device that allows a connection to be established as necessary and terminated when there is no longer a session to support. switch card Card on the LightStream 2020 ATM switch that handles communication between the other cards on the switch. Each LightStream 2020 switch has one or two switch cards. The second card, if present, serves as a backup for the first. switched LAN LAN implemented with LAN switches. See LAN switch. Switched Multimegabit Data Service See SMDS. Switched Port Analyzer See SPAN. switched virtual circuit See SVC. switched virtual connection See SVC. Switch Processor See SP. Switch-to-Switch Protocol See SSP. SwitchVision Cisco SNMP-based network management software, running on Microsoft Windows, that offers a powerful set of tools to manage an entire network, including switches, hubs, routers, and bridges. SwitchVision can automatically discover and map any SNMP device on the network and show the status of network devices. SwitchVision allows network administrators to set event thresholds, activate actions when error conditions occur, and set up custom tables and graphs to view critical network variables. synchronization Establishment of common timing between sender and receiver. Synchronous Data Link Control See SDLC. Synchronous Digital Hierarchy See SDH. Synchronous Optical Network See SONET. synchronous transmission Term describing digital signals that are transmitted with precise clocking. Such signals have the same frequency, with individual characters encapsulated in control bits (called start bits and stop bits) that designate the beginning and end of each character. Compare with asynchronous transmission, isochronous transmission, and plesiochronous transmission. Synchronous Transport Module level 1 See STM-1. Synchronous Transport Signal level 1 See STS-1. Synchronous Transport Signal level 3, concatenated See STS-3c. sysgen System generation. Process of defining network resources in a network. system generation See sysgen. system services control points See SSCP. Systems Network Architecture See SNA.